1. Field of the Invention
The present invention relates to the generation of a constant current, and particularly to a constant current circuit having a constant current characteristic in substantially no temperature dependence, and a constant current generating method.
2. Description of Related Art
FIG. 6 shows a constant current circuit according to a related art. Resistive elements R1 and R2 are series-connected between a predetermined voltage V0 and a ground potential. A division point of both resistive elements is connected to one input terminal of an amplifier. The other input terminal of the amplifier is connected to a source terminal of an NMOS transistor N1. The source terminal of the NMOS transistor N1 is connected to the ground potential via a resistive element R3. A gate terminal of the NMOS transistor N1 is connected to an output terminal of the amplifier. A drain terminal of the NMOS transistor N1 corresponds to an output terminal of the constant current circuit.
A predetermined voltage V0 is divided by the resistive elements R1 and R2, and a voltage V1 divided at the division point therebetween is inputted to the amplifier (V1=V0×R2/(R1+R2)). A signal outputted from the amplifier is applied to the gate terminal of the NMOS transistor N1, and a voltage applied to its source terminal is fed back to the other input terminal of the amplifier, whereby the voltages become approximately identical to each other between the input terminals of the amplifier. That is, the voltage V2 at the source terminal of the NMOS transistor N1 is controlled so as to be approximately equal to the divided voltage V1 (V2=V1). The voltage V2 is applied to the resistive element R3 so that an output current I is determined (I=V2/R3).
Here, the voltage V2 is equivalent to a voltage (V2=V1=V0×R2/(R1+R2)) which is approximately equal to the divided voltage V1 and obtained by dividing the predetermined voltage V0 by the resistive elements R1 and R2. If the predetermined voltage V0 is assumed to be a voltage in substantially no temperature dependence, which is generated by an unillustrated constant voltage generating circuit or the like, then the divided voltage V1 generated based on the ratio between the resistance values of the resistive elements R1 and R2 can be brought to a temperature dependence-cancelled characteristic even though the resistance values of the resistive elements R1 and R2 have temperature dependence respectively. Thus, the output current I obtained by applying the voltage V2 in substantially no temperature dependence to the resistive element R3 can be set as an output current for the constant current circuit.
A constant current generating circuit configured with bipolar transistors included therein has been disclosed in Japanese examined utility model application publication No. H7 (1995)-49537. A technique has been disclosed therein which is provided with resistive elements each having temperature dependence opposite to that of the bipolar transistor and cancels out temperature dependence at an output current. A voltage corresponding to a base-to-emitter voltage of the bipolar transistor having predetermined temperature dependence is applied to the corresponding resistive element whose resistance value has opposite temperature dependence, thereby to cancel out temperature dependence of a current that flows through the resistive element.